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It's written on the MDN that
If you want to pass parameters to the listener function, you may use an anonymous function.
After some experimentation I figured out when I try to register an event listener with a one-parameter function like this (without an anonymous function)
target.addEventListener(type, doSomething(parameter));
the listener function executes even when an event didn't happen, but when I wrap it up in an anonymous function
target.addEventListener(type, function () {doSomething(parameter);});
everything goes as expected.
Why does such behavior take place? I guess it is somehow connected with closures.
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To call removeEventListener with an anonymous function with JavaScript, we assign the anonymous event listener function to a variable and call addEventListener and removeEventListener with the variable. const onScroll = () => { //... }; document. body.
Event Listener BasicsTo set up an event listener you just need to have a variable that references an element and then call the addEventListener function on that element. This function takes a minimum of two parameters. The first parameter is just a string which is the name of the event to listen to.
In Python, an anonymous function is a function that is defined without a name. While normal functions are defined using the def keyword in Python, anonymous functions are defined using the lambda keyword. Hence, anonymous functions are also called lambda functions.
Strictly speaking you can't remove an anonymous event listener unless you store a reference to the function. Since the goal of using an anonymous function is presumably not to create a new variable, you could instead store the reference in the element itself: element. addEventListener('click',element.
I personally use a variable to store the <target> and declare the function outside of the event listener invocation eg: Then to remove the listener: Not the top recommendation you will receive but to remove anonymous functions the only solution I have found useful is to remove then replace the HTML element.
You can easily remove an event listener by using the removeEventListener () method. The first parameter is the type of the event (like " click " or " mousedown " or any other HTML DOM Event .) The second parameter is the function we want to call when the event occurs.
Add an Event Handler to the window Object The addEventListener () method allows you to add event listeners on any HTML DOM object such as HTML elements, the HTML document, the window object, or other objects that support events, like the xmlHttpRequest object.
The EventListener interface represents an object that can handle an event dispatched by an EventTarget object. Note: Due to the need for compatibility with legacy content, EventListener accepts both a function and an object with a handleEvent () property function.
When defining the handler function like so
target.addEventListener(type, doSomething(parameter));
You are passing the function's return value as handler. For example consider this function:
function doSomething(event) {
return 'foo';
}
Now, the function gets executed immediatly, before the event has happened, and you are basically just passing this as handler:
target.addEventListener(type, 'foo');
That can't work.
The second example
target.addEventListener(type, function () {doSomething(parameter);});
correctly passes a function as reference, without having it executed before the event occurred.
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